The present invention relates to the art of heat treating, and in particular, to a method and apparatus for sizing thin walled parts during a heat treating cycle.
The present invention is particularly applicable to altering irregular internal surfaces of thin walled parts formed by broaching and will be described with reference thereto; however, it will be hereinafter apparent that the invention has much broader aspects and may be beneficially used to control exterior surfaces and lengths as well, in parts formed by other machining operations such as turning, milling, shaping, boring and the like.
Broaching is widely employed in forming irregularly shaped surfaces where high production rates, close tolerances and smooth surfaces are required. The complete machining of the part can be performed accurately in a single pass of the broaching tool. This results in less expensive and faster production of parts than can be obtained by other conventional multi-step operations. For many complex shapes such as articles having internal cam surfaces, broaching is the only practical method for achieving the required accuracy in shape and size. During the broaching stroke, all of the cutting edges of the broach are cutting at once resulting in large tool forces, as severe as encountered in any other type of machining operation. Accordingly, the cutting teeth may break or wear, requiring frequent replacement or resharpening. As a consequence, subsequently produced parts may vary dimensionally with respect to previously produced parts and/or parts produced in parallel production lines. The broached parts may therefore have a range of dimensional sizes, some of which may be outside the product specification, and accordingly unacceptable, and others spanning an acceptable dimensional range.
In thin-wall, overrunning clutch cams in particular, in addition to requiring accurate control of the cam surfaces, the surfaces also must be heat treated for extended wear properties. By virtue of the metallurgical changes during the heat treating cycle, parts which, as machined, were within acceptable specifications may not be acceptable following the heat treating cycle. Inasmuch as further machining operations are impractical on the heat treated parts, further constraints are placed on the accuracy to which these parts must be broached to ensure that the post heat treated parts will have acceptable dimensions.